1. Field of the Invention
The present invention relates to a controller for a direct-injection internal combustion engine to be mounted on a vehicle or the like and a method of controlling the direct-injection internal combustion engine. More specifically, the present invention relates to a controller for a direct-injection internal combustion engine and a method of controlling the direct-injection internal combustion engine that, when the internal combustion engine is started under very low temperature conditions, control the ignition cut operation in which ignition of a mixture in a combustion chamber performed by the ignition plug is interrupted.
2. Description of the Related Art
In a direct-injection internal combustion engine (hereinafter also referred to as the direct-injection engine), a fuel injection valve (injector) is provided for each cylinder. The fuel, such as gasoline, is directly injected into combustion chambers via the fuel injection valves, and mixed with intake air introduced from inlet ports into the combustion chambers to form a mixture, which is ignited by ignition plugs. The direct-injection engine is excellent because of low fuel consumption, low emission and high power output. For this reason, the demand for the engine is rapidly increasing.
However, in the direct-injection engine, the time period from when the fuel is injected to when the fuel reaches the ignition point is short as compared to that of a port-injection engine that injects the fuel into the inlet port. For this reason, when the engine is started at a very low temperature (below −25° C., for example), atomization of the fuel injected into the cylinders becomes insufficient. As a result, ensured ignition cannot be achieved, and the startability becomes unstable.
As a measure against such a problem that arises when the engine is started under very low temperature conditions, there is engine-start control in which, when cranking is performed at the time of engine startup, the ignition cut operation is performed that inhibits ignition and performs fuel injection only, and in which, after the ignition cut operation is finished, both of fuel injection and ignition are performed (see Japanese Patent Application Publication No. JP-A-2000-097071, for example). If such the ignition cut operation is performed, part of the fuel injected during the ignition cut operation remains in the cylinders even after an exhaust stroke. Accordingly, by the time when the first ignition is initiated after the ignition cutoff period has elapsed, the fuel remaining in the cylinder atomizes. As a result, ignition is ensured.
As a technology concerning the ignition cut operation at the time of engine startup, there is a technology for optimizing the length of the ignition cutoff period depending on the engine temperature measured at the time of engine startup, by setting the period of time required for the fuel to atomize as the ignition cutoff period at an early stage of engine startup, based on the coolant temperature (see Japanese Patent Application Publication No. JP-A-11-270387, for example).
With regard to the direct-injection internal combustion engines that perform the ignition cut operation, in some cases, after the engine is stopped immediately after the engine is started under very low temperature conditions, it becomes difficult to start the engine when it is attempted to restart the engine with ignition cutoff performed. For example, when the inside of the cylinders is warmed up after the engine is started under very low temperature conditions, the fuel that can contribute to combustion increases because, in the state where the inside of the cylinder has been warmed up, the fuel injected when the engine is restarted is warmed up due to the temperature in the cylinders. Under such conditions, if the ignition cut operation is performed when the engine is restarted after the engine is stopped (when the engine is restarted in a state where the inside of the cylinders has been warmed up), there is the fuel that can be effectively used in the cylinder, and, on the top of that, the fuel provided during the ignition cut operation is added in the cylinders. Accordingly, the fuel condition becomes over-rich, and “wetting of ignition plugs” and/or “smoldering of ignition plugs” are caused. As a result, it becomes difficult to start the engine. It should be noted that the state where the inside of the cylinders has been warmed up does not mean a state where the engine has been warmed up to the extent that the coolant temperature becomes 80° C. or more (the engine has been completely warmed up), for example, but means a state where the inside of the cylinders has been warmed up to the extent that the fuel becomes easy to atomize.
When the engine is started under very low temperature conditions, if the engine is stopped (cranking is stopped) during the ignition cut operation, or if the engine is stopped before the fuel has been completely burned, it can become difficult to start the engine when the engine is restarted after the engine is stopped as described above. That is, if the engine is stopped during the ignition cut operation, the fuel injections into the cylinders corresponding to the number of times of ignition cut operation performed by the time when the engine is stopped, have been performed. If, thereafter, the ignition cut operation is performed a predetermined number of times at the time of restart, in addition to the injection performed during the ignition cut operation last time, the fuel injection is further performed during the ignition cut operation performed this time. Also in this case, the fuel condition becomes over-rich, and it can become difficult to start the engine.
With regard to 6-cylinder engines, for example, in the case where the requested number of times of ignition cut operation is twelve, if, during the ignition cut operation at the time of engine startup under very low temperature conditions, the engine is stopped at the time when the ignition cut operation has been performed nine times, and if the ignition cut operation is performed twelve times at the time of restart (at the time of startup under very low temperature conditions), the total number of times the fuel is injected during the ignition cut operation is 21. Accordingly, “wetting of ignition plugs” and/or “smoldering of ignition plugs” can occur due to the over-rich fuel condition.
The problem of the over-rich fuel condition caused when the engine is restarted under very low temperature conditions while the inside of the cylinders is in a warmed-up state, or when the engine is restarted under very low temperature conditions after the engine is stopped during the ignition cut operation, is not considered in Japanese Patent Application Publication No. JP-A-11-270387 cited above. The above-described problems cannot be solved by the method described in Japanese Patent Application Publication No. JP-A-11-270387, that is, the method in which the period of time required to atomize the fuel is set as the ignition cutoff period at an early stage of startup, based on the coolant temperature, with attention being focused only on the time required for the injected fuel to atomize.